U.S. patent number 6,450,968 [Application Number 09/654,480] was granted by the patent office on 2002-09-17 for method for determining gas content in a breathing apparatus, and a breathing apparatus operating according to the method.
This patent grant is currently assigned to Siemens Elema AB. Invention is credited to Rolf Castor, Lars Wallen.
United States Patent |
6,450,968 |
Wallen , et al. |
September 17, 2002 |
Method for determining gas content in a breathing apparatus, and a
breathing apparatus operating according to the method
Abstract
In a method for determining the gas content, e.g. of oxygen in
breathing gas in a breathing apparatus, and a breathing apparatus
operating according to the method, the gas content is determined
from the speed of sound in the breathing gas. In order to resolve
the problems caused by temperature variations in gas samples,
determination of the speed of sound is synchronized with one or
more specific times in a respiratory cycle. Determination can then
be made when conditions are most stable.
Inventors: |
Wallen; Lars (Sp.ang.nga,
SE), Castor; Rolf (Hagersten, SE) |
Assignee: |
Siemens Elema AB (Solna,
SE)
|
Family
ID: |
20416910 |
Appl.
No.: |
09/654,480 |
Filed: |
September 1, 2000 |
Foreign Application Priority Data
Current U.S.
Class: |
600/532; 422/84;
73/23.3 |
Current CPC
Class: |
A61B
5/087 (20130101); G01N 29/024 (20130101); G01N
29/38 (20130101); G01N 2291/0215 (20130101); G01N
2291/02466 (20130101); G01N 2291/02809 (20130101); G01N
2291/02881 (20130101) |
Current International
Class: |
A61B
5/087 (20060101); A61B 5/08 (20060101); G01N
29/02 (20060101); G01N 29/36 (20060101); G01N
29/38 (20060101); G01N 29/024 (20060101); A61B
005/00 () |
Field of
Search: |
;600/529-538 ;73/23.3
;422/84 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nasser; Robert L.
Attorney, Agent or Firm: Schiff Hardin & Waite
Claims
We claim as our invention:
1. A method for determining a gas content of a gas component of
breathing gas in a breathing apparatus, comprising the steps of:
connecting a breathing apparatus to a patient exhibiting a
respiratory cycle; making at least one measurement of a speed of
sound in said breathing gas and determining a gas content in said
breathing gas from the speed of sound therein; and synchronizing
measurement of the speed of sound in said breathing gas with at
least one predetermined time in said respiratory cycle.
2. A method as claimed in claim 1 comprising measuring the speed of
sound in the breathing gas within a specific interval preceding an
inspiration phase in said respiratory cycle.
3. A method as claimed in claim 1 comprising measuring the speed of
sound in the breathing gas within a specific interval preceding an
expiration phase in said respiratory cycle.
4. A method as claimed in claim 1 comprising measuring the speed of
sound in the breathing gas a plurality of times to obtain a
plurality of speed of sound measurements, and comparing said
plurality of speed of sound measurements to determine a trend of
said gas content.
5. A method as claimed in claim 4 comprising accepting said
determination of said gas content from said plurality of
measurements of the speed of sound in said breathing gas, if said
trend is stable.
6. A method as claimed in claim 4 comprising approximating said gas
content in said breathing gas from said plurality of measurements
of the speed of sound in said breathing gas if said trend is not
stable.
7. A method as claimed in claim 6 comprising approximating said gas
content by exponential regression.
8. A breathing apparatus comprising: an inspiration line and an
expiration line adapted for connection to a patient exhibiting a
respiratory cycle wherein breathing gas is adapted to be carried to
a patient through said inspiration line during an inspiration phase
of said respiratory cycle and wherein breathing gas is conducted
away from a patient through said expiration line during an
expiration phase of said respiratory cycle; and a gas meter in
fluid communication with at least one of said inspiration line and
said expiration line for determining a gas content of at least one
gas component in said breathing gas, said gas meter determining a
speed of sound in said breathing gas synchronized with at least one
predetermined time in said respiratory cycle.
9. A breathing apparatus as claimed in claim 8 wherein said gas
meter comprises an elongated measurement chamber having an inlet
and an outlet both connected to said inspiration line and an
ultrasound mete, for ultrasonically measuring the speed of sound in
said inspiration line, a temperature unit for determining a
temperature of said breathing gas in said inspiration line, and an
evaluation unit supplied with the speed of sound from said
ultrasound meter and with the temperature from said temperature
unit for determining said gas content from said speed of sound and
from said temperature.
10. A breathing apparatus as claimed in claim 9 wherein said gas
meter comprises a first sintered filter disposed at said inlet and
a second sintered filter disposed at said outlet, and a mesh
disposed in said inspiration line between said first sintered
filter and said second sintered filter.
11. A breathing apparatus as claimed in claim 9 wherein said gas
meter comprises a first valve disposed at said inlet and a second
valve disposed at said outlet for controlling a flow of said
breathing gas through said measurement chamber.
12. A breathing apparatus as claimed in claim 9 comprising a second
gas meter disposed in said inspiration line for measuring a further
gas component in said breathing gas in said inspiration line, said
second gas meter being connected to said evaluation unit.
13. A breathing apparatus as claimed in claim 8 wherein said gas
meter comprises an elongated measurement chamber having an inlet
and an outlet both connected to said expiration line and an
ultrasound meter for ultrasonically measuring the speed of sound in
said expiration line, a temperature unit for determining a
temperature of said breathing gas in said expiration line, and an
evaluation unit supplied with the speed of sound from said
ultrasound meter and with the temperature from said temperature
unit for determining said gas content from said speed of sound and
from said temperature.
14. A breathing apparatus as claimed in claim 13 wherein said gas
meter comprises a first sintered filter disposed at said inlet and
a second sintered filter disposed at said outlet, and a mesh
disposed in said expiration line between said first sintered filter
and said second sintered filter.
15. A breathing apparatus as claimed in claim 13 wherein said gas
meter comprises a first valve disposed at said inlet and a second
valve disposed at said outlet for controlling a flow of said
breathing gas through said measurement chamber.
16. A breathing apparatus as claimed in claim 13 comprising a
second gas meter disposed in said expiration line for measuring a
further gas component in said breathing gas in said expiration
line, said second gas meter being connected to said evaluation
unit.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for determining the gas
content in a breathing apparatus as well as to a breathing
apparatus operating according to the method.
2. Description of the Prior Art
Accurate regulation of the oxygen content of breathing gas supplied
to a patient with a breathing device is vitally important. Oxygen
content is measured by an oxygen meter to ensure that the delivered
oxygen content is correct. If the oxygen content of expired gas is
also measured, the patient's consumption of oxygen can be
established. Important information can also be derived from the
amount of expired carbon dioxide. In other contexts, determination
and monitoring of the concentration of other gases supplied, such
as helium, also may be desirable.
One known method for determining the content of a gas component in
a binary gas mixture (e.g. the oxygen content of a mixture of air
and oxygen) involves determination of the speed of sound in the
mixture. The speed of sound is usually measured with ultrasound,
however, this type of measurement is temperature-related, since the
speed of sound is temperature-related. The temperature of the
mixture therefore is usually measured as well (or the temperature
in a test chamber and test sample is regulated in such a way that a
known temperature is maintained.)
Temperature variation is one problem encountered in the use of an
ultrasonic-type gas meter in breathing apparatuses. The oxygen
content of delivered breathing gas should be measured in every
respiratory cycle. During inspiration, gas flows through the
inspiration line at high speed for a relatively brief period.
Temperature variations are therefore very striking and pose
difficulties in simultaneous determination of temperature and air
speed in an accurate and reliable manner.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for
determining the gas content of breathing gas in breathing devices
which avoids the aforesaid problems.
Another object of the invention is to provide a breathing apparatus
in which correct measurement of the gas content of breathing gas
can be made.
The above object is achieved in accordance with the principles of
the present invention in a method for determining a gas content in
a breathing apparatus, and in a breathing apparatus operating
according to the method, wherein the gas content is determined from
the speed of sound in the gas, and wherein the determination of the
speed of sound is synchronized with one or more specific times in a
respiratory cycle.
By synchronizing measurements of the speed of sound with specific
times in respiratory cycles, measurement can be made when the
temperature is stable and easy to measure/monitor.
Performing the measurement immediately before an inspiration
commences is particularly advantageous in the determination of the
gas content of breathing gas for delivery to a patient. Conditions
in the inspiration section of the breathing apparatus are then most
stable. The fact that this causes a de facto one-breath lag in
measurement of e.g. oxygen content does not pose any risk to the
patient. At worst, she/he only receives one breath with a
(partially) erroneous gas composition.
In a corresponding manner, the gas content in expired air can be
determined by measurement before an expiration phase begins.
Measurement of the content of both inspired and expired gas
supplies information on the patient's uptake. Compensation must be
made in this measurement, however, for the patient's contribution
of carbon dioxide to the gas mixture. This is easily achieved with
a carbon dioxide meter installed next to the oxygen meter.
Measurement can be made by pulsing ultrasound at a certain clock
frequency in order obtain a series of measurement values at every
determination of oxygen content. The measurement values can be used
for identifying a trend.
If the trend is stable (the same results are obtained at every
measurement point), the determined oxygen content can be accepted
as correct.
If the trend is unstable, i.e. measurement values vary, the oxygen
content can still be estimated by analyzing the measurement values.
Exponential regression is a known method for such an analysis. It
yields a sufficiently accurate approximation of oxygen content.
In an embodiment of the apparatus, the oxygen meter incorporates a
measurement chamber with an inlet and outlet connected to an
inspiration line. The inlet and outlet are equipped with sintered
filters, causing heat to be stored, so the temperature in the
measurement chamber is kept more constant. A slight drop in
pressure, large enough to divert part of the flow during
inspiration into the measurement chamber to replace the gas sample,
is created with the aid of a mesh in the inspiration line.
In an alternative embodiment, valves are placed at the inlet and
the outlet to control the exchange of gas samples.
DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an embodiment of a breathing apparatus according to
the invention.
FIG. 2 shows a first embodiment of an oxygen meter in the inventive
breathing apparatus.
FIG. 3 is a diagram for estimating oxygen content in an embodiment
of the method according to the invention.
FIG. 4 shows a second embodiment of an oxygen meter in the
inventive breathing apparatus.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a breathing apparatus 2 connected to a patient 4 in
order to supply her/him with a breathing gas. The breathing gas is
delivered to the breathing apparatus through a first gas connection
6A for air and a second gas connection 6B for oxygen. The
proportions of air and oxygen are regulated in a valve unit 8
according to a pre-set oxygen concentration (21% to 100%). The
valve unit 8 also regulates the pressure and flow of the breathing
gas.
Other gases can naturally be connected to the breathing apparatus
2, e.g. oxygen-helium, air-carbon dioxide, etc.
The breathing gas is carried to the patient 4 in an inspiration
line 10 and evacuated from the patient 4 in an expiration line 12.
Here, an expiration valve 14 regulates e.g. end-expiratory pressure
(PEEP) etc.
All functions in the breathing apparatus 2 are controlled by a
control unit 16 in the known fashion. Here, the breathing apparatus
2 can be e.g. a Servo Ventilator 300 from Siemens-Elema AB,
Sweden.
A first gas meter 18 is connected to the inspiration line 10 for
determining the oxygen content of supplied breathing gas. A second
gas meter 20 can be connected to the expiration line 12 for
determining the content of expired gas. The gas meters 18, 20 can
be oxygen meters for the air-oxygen gas mixture. For other gas
mixtures, e.g. oxygen-helium, the gas meters 18, 20 can be helium
meters. The measurement principle is the same, regardless of the
gas mixture, thus the method is described below for measurement of
oxygen.
FIG. 2 shows an embodiment of the first oxygen meter 18. The first
oxygen meter 18 contains a measurement chamber 22 that is parallel
to the inspiration line 10. A gas sample can be sent to the
measurement chamber 22 through an inlet 24 which is, in this case,
an orifice in which a first sintered filter 24A is arranged. The
gas sample leaves the measurement chamber 22 through an outlet 26
consisting, in this case, of an orifice in which a second sintered
filter 26A is arranged.
The gas sample in the measurement chamber is replaced at every
inspiration. This is accomplished when a mesh 28 (or some other
resistance) is arranged in the inspiration line 10 between the
inlet 24 and the outlet 26. Pressure across the mesh 28 drops when
breathing gas flows through the inspiration line 10. This diverts a
small part of the inspiration flow into the inlet 24 (at the same
time as the previous gas sample departs from the measurement
chamber 22 through the outlet 26).
An ultrasonic transmitter/receiver 30 is arranged in the
measurement chamber for measuring the speed of sound in the gas
sample in the measurement chamber 22. The sound is emitted by the
ultrasonic transmitter/receiver 30, bounces off the opposite wall
of the measurement chamber 22 and returns to the ultrasonic
transmitter/receiver 30. The speed of sound is determined in an
evaluation unit 32. When measurements are to be made, the speed of
sound can be determined e.g. every millisecond throughout the
measurement procedure. This yields a series of measurement results
all of which should be identical.
The speed of sound in the binary gas mixture of air and oxygen is
related to the proportions of the two gases. Therefore, the
composition (primarily the oxygen content) can be established by
determining the speed of sound in the gas sample.
A temperature sensor 34 for determining the temperature is also
arranged in the measurement chamber 22. Temperature information is
sent, via an amplifier 36, to the evaluation unit 32. In the
determination of the speed of sound (and accordingly the oxygen
content) the prevailing temperature is taken into account.
Measurement of the oxygen content is made immediately before the
inspiration phase begins in order to obtain the best possible
measurement results. At that point, the temperature of the gas
sample and the thermal sensor 34 have had the maximum time to
stabilize.
This poses no problems in controlled respiration in which the
inspiration phases are activated by the breathing apparatus 2. In
supported respiration or spontaneous respiration, the start of the
inspiration phase can be estimated from previous respiratory
cycles. Alternatively, measurement can be made with somewhat wider
margins and performed in the final phase of expiration, e.g. when
flow in the expiration line 12 has dropped to a specific level.
Other ways of determining a time before the inspiration phase are
also conceivable.
As noted above, a number of measurements of the speed of sound (and
accordingly oxygen content) can be made on each determination
occasion (respiratory cycle). Measurement values can be used for
establishing a trend.
As long as the trend is stable (oxygen content is constant for each
measurement value with only a small percentage of deviating
measurements), the measurement value can be accepted as a true
value.
If the trend is unstable (the oxygen content varies in each
measurement value), this may be indicate that conditions have not
had time to stabilize properly. In principle, this measurement
sequence can be ignored and a new measurement made in the next
respiratory cycle.
It is also possible to approximate the true oxygen content from a
trend analysis of the data obtained.
One such trend analysis is illustrated in FIG. 3, a diagram showing
oxygen content and time (measurement point). A few measurement
points 38 have been shown in FIG. 3 to illustrate estimation of the
true value for oxygen content. In principle, the distance between
each measurement point 38 can be about 1 millisecond.
Using e.g. exponential regression, a curve 40 can be established
for the measurement values. The curve 40 asymptotically approaches
a line 42 used as an approximated value for the true oxygen
content.
A second embodiment of the gas meter is shown in FIG. 4. In this
case, the gas meter 20 connected to the expiration line 12 is
shown. The gas meter 20 has an elongated measurement chamber 44
arranged alongside the expiration line 12. Via a first valve 46 and
a second valve 48 gas sample within the chamber 44 can be replaced
by opening the valves 46, 48 as indicated with dotted lines.
Preferably, the valves are constructed, so that essentially the
entire flow in the expiration line 12 will pass through the chamber
44. By selecting the time when the valves are open, the gas sample
obtained can be taken from any part of the breathing cycle. In the
present embodiment, a gas sample for instance could be taken from
the first part of expiration or from a latter part of the
expiration. The valves 46, 48 need only be open as long as it takes
to exchange the gas sample within the chamber 44.
A thermostat 50 is arranged within the chamber to provide a
constant temperature on the sample. It may be preferable to have a
temperature close to the gas samples temperature in order to speed
up the time it takes to reach a uniform temperature in the chamber
44.
As in the first embodiment, an ultrasonic transmitter/receiver 30
is arranged in the chamber 44 for measuring the speed of sound in
the gas sample and the evaluation unit 32 determines gas
content.
In the second embodiment, the determination unit 32 could also
include control means to control the valves 46, 48 (not shown in
the figure). In the alternative, the valves can be controlled by a
separate control unit or other means of control, dependent on the
breathing cycles.
Since measurement in the second embodiment is made on a gas mixture
that can contain three gases (for instance oxygen, nitrogen and
carbon dioxide), a further measurement is required to make accurate
calculations of e.g. oxygen content. A further gas meter 52 is
arranged in or connected to the chamber 44. In this case, the
further gas meter 52 measures content of carbon dioxide. A
measurement signal indicating content of carbon dioxide is sent to
the determination unit 32. The determination of gas content based
on ultrasound is then compensated with known content of carbon
dioxide.
Other versions of the aforementioned exemplifying embodiments are
also conceivable. The gas meter according to the first embodiment
can be used to measure gas content on the expiration side and the
gas meter according to the second embodiment can be used to
determine gas content on the inspiration side. Moreover, many
combinations of the two embodiments are possible. For example, the
temperature sensor 34 can be replaced with a thermostat 50 for
maintaining a constant temperature in the measurement chamber. The
sintered filters 24A, 26A can be replaced with other kinds of
filters or left out altogether. The sintered filters 24A, 26A can
also be used in combination with the valves 46, 48 in order to more
rapidly achieve a stable environment in the measurement chamber
22,44. The important feature of the invention is synchronization of
the determination of gas component content with specific times in
respiratory cycles.
Although modifications and changes may be suggested by those
skilled in the art, it is the intention of the inventors to embody
within the patent warranted hereon all changes and modifications as
reasonably and properly come within the scope of their contribution
to the art.
* * * * *